Liquid crystal display (LCD) devices use the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. The liquid crystal molecules have long, thin, shapes, and have an initial alignment direction including initial pretilt angles. The alignment direction can be controlled by applying an electric field to influence the alignment of the liquid crystal molecules. Due to an optical anisotropy property of liquid crystal, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by properly controlling the applied electric field, an image having a desired brightness can be produced.
Among the known types of liquid crystal displays (LCDs), active matrix LCDs (AM-LCDs), which have thin film transistors (TFTs) and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superior ability in displaying moving images.
LCD devices include two substrates spaced apart and facing each other, and a liquid crystal layer interposed between the two substrates. In one type of LCD device, each of the substrates includes an electrode with the electrodes of each substrate facing each other. A voltage is applied to each electrode inducing an electric field between the electrodes. The arrangement of the liquid crystal molecules is changed by varying the intensity of the electric field.
Because the electrodes are positioned respectively on each of the two opposing substrates, the electric field induced between the electrodes is perpendicular to the surfaces of the two substrates. Accordingly, LCD devices of this type have a narrow viewing angle because of the vertical electric field. In order to solve the problem of the narrow viewing angle, in-plane switching mode liquid crystal display (IPS-LCD) devices have been proposed. An IPS-LCD device includes a pixel electrode and a common electrode on the same substrate.
FIG. 1 is a cross-sectional view illustrating an IPS-LCD device according to the related art. In FIG. 1, an IPS-LCD device 5 includes a first substrate 10 and a second substrate 40 with a liquid crystal layer LC interposed therebetween. A pixel region P is defined on the first substrate 10. A thin film transistor (TFT) T is formed in the pixel region P on the first substrate 10 for use as a switching element. Common electrodes 18 and pixel electrodes 32 are also formed in the pixel region P. The TFT T includes a gate electrode 12, a semiconductor layer 22, a source electrode 24, and a drain electrode 26. A gate insulating layer 20 is formed between the gate electrode 12 and the semiconductor layer 22. The common electrodes 18 alternate with and are substantially parallel to the pixel electrodes 32 on the first substrate 10. The common electrodes 18 are formed of the same material and on the same layer as the gate electrode 12. A passivation layer 30 is formed on the TFT T and the pixel electrodes 32 are formed on the passivation layer 30. To increase aperture ratio and brightness, the pixel electrodes 32 may be formed of a transparent conductive material the same material and on the same layer as the source and drain electrodes 24 and 26.
The second substrate 40 is spaced apart from the first substrate 10. A black matrix 42 is formed on an inner surface of the second substrate 40 facing the first substrate 10. The black matrix 42 on the second substrate 40 corresponds to the TFT T, the gate line and the data line on the first substrate 10. A color filter layer 44 including three color filters of red 44a, green 44b, and blue (not shown) is formed on the black matrix 42. The color filter layer 44 corresponds to the pixel region P on the first substrate 10. The liquid crystal layer LC is interposed between the first substrate 10 and the second substrate 40. The alignment of the liquid crystal layer LC is controlled by a horizontal electric field induced between the common electrode 18 and the pixel electrode 32.
To improve aperture ratio and brightness further, an IPS-LCD device having common electrodes of a transparent conductive material has been suggested. FIG. 2 is a schematic plan view of an array substrate for an IPS-LCD device according to the related art and FIG. 3 is a schematic cross-sectional view taken along a line III-III of FIG. 2. As shown in FIGS. 2 and 3, a gate line 52 and a data line 68 are formed on a substrate 50. The gate line 52 and the data line 68 cross each other to define a pixel region P. A thin film transistor (TFT) T is connected to the gate line 52 and the data line 68. The TFT T includes a gate electrode 54, an active layer 60, a source electrode 64 and a drain electrode 66. A pixel electrode 72 and a common electrode 74 are formed of a transparent conductive material in the pixel region P. The pixel electrode 72 includes a horizontal portion 72a and vertical portions 72b extending from the horizontal portion 72a, and the common electrode 74 includes a horizontal portion 74a and vertical portions 74b extending from the horizontal portion 74a. 
In addition, an auxiliary common electrode 56 having a rectangular ring shape is formed in the pixel region P. Accordingly, the auxiliary common electrode 56 includes first and second horizontal portions 56a and 56b and first and second vertical portions 56c and 56d constituting the rectangular ring shape. The common electrode 74 contacts the auxiliary common electrode 56, and the pixel electrode 72 contacts the drain electrode 66. As a result, a common voltage is applied to the common electrode 74 through the auxiliary common electrode 56 and a data signal is applied to the pixel electrode 72 through the drain electrode 66.
The first horizontal portion 56a of the auxiliary common electrode 56 overlaps the horizontal portion 72a of the pixel electrode 72 to constitute a storage capacitor Cst. Overlapped portions of the first horizontal portion 56a and the horizontal portion 72a function as first and second capacitor electrodes, respectively, of the storage capacitor Cst. The storage capacitor includes a gate insulating layer 58 and a passivation layer 70 between the first and second capacitor electrodes as a dielectric layer.
The gate insulating layer 58 is interposed between the gate line 52 and the data line 68 to prevent shortage and signal interference of the gate line 52 and the data line 68. For example, the gate insulating layer 58 may have a thickness of about 4000 Å. The passivation layer 70 is formed on the TFT T to protect the active layer 60 exposed between the source and drain electrodes 64 and 66. The passivation layer 70 further protects the source and drain electrodes 64 and 66. For example, the passivation layer 70 may have a thickness of about 2000 Å. Accordingly, the dielectric layer of the storage capacitor Cst may have a thickness of about 6000 Å. Since a capacitance of the storage capacitor Cst is inversely proportional to a thickness of the dielectric layer and is proportional to an area of the electrode, increase in the area of the electrode is required to obtain a higher capacitance at a given thickness of the dielectric layer. However, as the area of the electrode increases, aperture ratio and brightness of the IPS-LCD device decreases. In addition, a high resolution is not obtained in the IPS-LCD device.